Numerous methods and apparatus exist in the art for measuring chemical components of fluids and current technology utilizes many types of sensors for detecting components and analytes in numerous types of fluids. For example, carbon dioxide and pH sensors are used for measuring these components in various fluids including gases and liquids. For instance, the measurement of blood gases, along with the pH from a sample of arterial blood, gives the state of the acid base balance or the effectiveness of both the respiratory and cardiovascular systems of the human or vertebrate body. Measuring the blood gases usually involves a measurement of the partial pressures of oxygen and carbon dioxide along with the measurement of the pH since carbon dioxide dissolved in the aqueous solution can affect the pH through the presence of carbonic acid. These are examples of coupled analytes.
It is conventional practice in many of the existing measurement methods, even where the fluid is a liquid or liquid with a dissolved gas with or without the presence of solids to transport a sample to a central location for testing. With centralized testing, the bulky, stationary, elaborate and sophisticated equipment performs the analysis on a practically endless number of samples. Originally, this equipment employed carbon dioxide sensor like the Severinghaus potentiometric sensor for a qualitative and/or quantitative measure of carbon dioxide. This sensor has a gaspermeable membrane between the sample solution for measurement and the measuring cell. The cell has a pH-sensing glass electrode, a reference electrode, and an intermediate electrolyte layer. Recently, more sophisticated carbon dioxide sensors have utilized polymeric membranes like those of the combined pH and carbon dioxide sensor of U.S. Pat. No. 4,818,361.
Also, recent attempts have been made to introduce more portable equipment into the marketplace of fluid analysis. An example of this is the qualitative and/or quantitative measurement of constituents or analytes of blood. The bulky stationary equipment is fairly expensive and the procedures for its use can be cumbersome depending on the type of fluid to be measured. For instance, measuring blood gases from the arterial blood sample involves: drawing the blood sample in a syringe, immersing it in ice and transporting it quickly to the lab where the equipment is usually located for a measurement of the gases. More portable devices would shorten or overcome transporting the sample to the measuring equipment at a fixed location. For example, portable sensing units which can be coupled to a digital readout device would be useful at the patient's bedside in a manner similar to a way that temperatures are measured at the patient bedside.
U.S. Pat. Nos. 3,000,805 and 3,497,442 show two such devices. The former has electrodes located on a syringe plunger and the latter has electrodes placed on the syringe well to conduct the measurements. The electrodes of these sensors may be particularly sensitive to small sample volumes since they consume oxygen in their operation. In U.S. Pat. No. 5,046,496, Applicants' assignee describes and claims a portable blood gas sensor which includes sensors fabricated using a conventional silk screening process where the electrodes are screened on to a ceramic substrate. Typically, these electrodes have the conductor along with an electrolyte and analyte permeable polymeric membrane that covers the sensor. Some of these membranes may be hydratable membranes by water vapor permeation and they can be stored in a dry state and hydrated just prior to use as in U.S. Pat. No. 4,818,361. The more portable the equipment the larger the demand for the miniaturization of the electrodes that still produce precise outputs for the analyte concentration or tension.